1. Field of Use
This invention relates generally to gantries for spaning loads which are to be raised and lowered. In particular, the invention relates to gantries which comprise spaced-apart vertically adjustable side support assemblies having horizontal load-bearing gantry beam supported therebetween.
2. Description of the Prior Art
Large gantries of the aforesaid character are sometimes employed instead of boom-type cranes to emplace or remove large heavy loads on trucks, trailers and rail-road flat cars. Such gantries for handling loads weighing hundreds of tons may be on the order of 25 feet wide and 25 feet high and, because of their size and weight, are assembled on the job site. In some gantries the side support assemblies take the form of stationary structures, such as A-frames, and hoists are mounted on the gantry beam to raise and lower the load. However, in some other gantries the side support assemblies themselves can be raised and lowered vertically, as by means of a single vertically disposed extendable/retractable hydraulic ram, to raise and lower the load-bearing gantry beam and any load thereon. In gantries employing such hydraulic rams, a control system is provided to enable the gantry operator to extend and retract the rams in unison. A principal difficulty with such prior art hydraulic ram systems is that extreme ram extension to raise the beam and load thereon causes the entire gantry to become unstable and the effect is further aggravated as the loads become heavier. In extreme cases it is possible for the gantry to tip over thereby risking or causing damage to the gantry, the load and near-by equipment and injury to near-by personnel. It is necessary, therefore, to limit the weight of the load being handled, or to limit the height to which the side supports are raised, or to provide a larger gantry. These considerations and solutions are time-consuming and costly.